The Alliance for Telecommunication Industry Solutions (ATIS) defines “non-return-to-zero (NRZ)” to be a code in which logic “1s” are represented by one “significant condition” and logic “0s” are represented by another, with no neutral or rest condition, such as a zero amplitude in amplitude modulation (AM), zero phase shift in phase-shift-keying (PSK), or mid-frequency in frequency-shift-keying (FSK). The term “significant condition” is defined as follows. In the modulation of a carrier, “significant condition” is one of the values of the signal parameter chosen to represent information. Examples of “significant conditions” are an electrical current, voltage, or power level; an optical power level; a phase value; or a frequency or wavelength chosen to represent a “0” or a “1”; or a “mark” or a “space.” The duration of a significant condition is the time interval between successive “significant instants”.
Significant conditions are recognized by an appropriate device. Each “significant instant” is determined when the appropriate device assumes a condition or state usable for performing a specific function, such as recording, processing, or gating. A change from one significant condition to another is called a “signal transition”.
For a given data signaling rate (i.e., bit rate) the NRZ code requires only one-half the bandwidth required by the well-known “Manchester Code”. One skilled in the art will recognize that the Manchester Code is a code in which (a) data and clock signals are combined to form a single self-synchronizing data stream, (b) each encoded bit contains a transition at the midpoint of a bit period, (c) the direction of transition determines whether the bit is a “0” or a “1,” and (d) the first half is the true bit value and the second half is the complement of the true bit value.
The process of varying one or more parameters of a carrier wave as a function of two or more finite and discrete states of a signal is known as digital modulation. It is the process by which some characteristic (frequency, phase, amplitude, or combinations thereof) of a carrier frequency is varied in accordance with a digital signal, for example, a digital signal consisting of coded pulses or states.
One particular class of digital modulation employing NRZ is the one for which the time between signal transitions is a multiple of the embedded clock period. There are many practical and theoretical approaches of this sort to transmitting information. In such implementations the information data is transmitted or retrieved without any additional timing reference. Examples include the high-frequency waveforms for optical storage (CD/DVD), serial data communications standards (Infiniband, PCI Express), and many others. In some of these cases the information is contained in the timing between signal transitions (as in optical storage CD/DVD signals), in some others the information is contained in the amplitude of the signal between the signal transitions, and so on.
The salient feature of digital modulation using NRZ with integer numbers of clock periods between signal transitions is that for each clock period there is exactly one logic “1” or logic “0” bit of information associated with each clock period time length of decoded signal. Therefore, a major step in decoding these types of digital modulation signals is the recovery of the embedded clock, or the determination of the period of the embedded clock. The complexity of this task is greatly reduced if the period of the NRZ signal is first known.